This invention concerns a process for providing powders of thermoplastics such as polyethylene homopolymer, ethylene-vinyl acetate (EVA) copolymer and partially hydrolyzed ethylene-vinyl acetate (HEVA) copolymer by an aqueous dispersion procedure and, in particular, is an improvement of the process of U.S. Pat. No. 4,151,135, the entire contents of which are incorporated by reference herein.
Thermoplastic resins in finely-divided form have found use in a number of applications where it is either impossible or inconvenient to utilize the more conventional cube or pellet forms. For example, powdered organic polymeric thermoplastic resins in dry form have been used to coat articles by dip coating wherein the powder is applied by spraying or dusting, and by flame spraying. In dispersed form, thermoplastic resin powders have been applied as coatings by roller coating, spray coating, slush coating, and dip coating to substrates such as metal, paper, paperboard, and the like. These powders have also been widely employed in conventional powder molding techniques. Other applications of these powders include paper pulp additives; mold release agents for rubber; additives to waxes, paints, and polishes; binders for non-woven fabrics; and so on.
High pressure EVA copolymers are a well-known class of thermoplastic resins. Copolymers containing up to about 60 weight percent of vinyl acetate are now available commercially via a modified high pressure polyethylene process operating at 1,000-2,000 atmospheres, and compositions containing as much as 85 weight percent vinyl acetate have been made experimentally by the high pressure process. EVA resins containing up to about 35 weight percent of vinyl acetate are readily handled in the form of pellets, yet even resins with a vinyl acetate content in the upper part of this range tend to stick together under the pressure of their own weight, especially under hot storage conditions. Copolymers containing between about 35 and about 60 weight percent of vinyl acetate can also be pelletized, but the pellets tend to be tacky and coalesce increasingly at higher vinyl acetate contents in this range. At about 40 to 45 weight percent vinyl acetate, the pellets maintain their identity, but often partially fuse into bulky masses resembling bunches of grapes. At 50 to 60 weight percent vinyl acetate content, the pellets soon lose their identity and these resins normally assume the shape of their container by cold flow, and consequently are only available as solid blocks. Copolymers containing 35-85 weight percent vinyl acetate can be produced with melt flow rates below 5 (Condition B, ASTM D1238) and in most cases as low as 0.2, when finished by thermal treatment by known methods, e.g., U.S. Pat. No. 3,968,091.
U.S. Pat. No. 3,517,083 discloses that EVA resins containing 15 to 60 weight percent vinyl acetate may be used as impact modifiers in rigid polyvinyl chloride (PVC) formulations, and that EVA copolymers containing 60 to 85 weight percent vinyl acetate are useful in producing flexible blends with PVC. However, a serious impediment in blending high pressure EVA into PVC is the aforesaid physical form of the EVA resins. To blend even PVA pellets into PVC, which is normally supplied as a powder, requires the expenditure of considerable energy and introduces an undesirable heat history into the PVC (e.g., see Plastics Engineering, April 1967, p. 47; Plastics Technology, July 1975, P. 50). Blending of the fused pellets characteristic of the EVA resins containing above about 50 weight percent vinyl acetate would obviously be still more difficult than blending free pellets.
Accordingly, to facilitate the blending of high pressure EVA copolymers into PVC powder, and into other pulverulent polymers as well, it is often desirable to have the EVA copolymers in the powder form. Dry blending of PVC powder and the EVA powder is then readily accomplished at little expenditure of energy and without imparting an undesirable heat history to the heat-sensitive PVC due to the blending operation itself.
Hydrolyzed HEVA copolymers, particularly the so-called partially hydrolyzed copolymers, herein defined as EVA resins originally containing about 35 to about 85 weight percent of vinyl acetate, have been generally known for many years. They resemble the high pressure EVA resins in being inherently tacky materials but present certain advantages by virtue of their hydroxyl functionality, e.g., enhanced adhesion to various substrates, additional cross-linkability, and have superior heat and mill stability as compared to EVA. In addition, as disclosed in this invention, those less than 50% hydrolyzed also function as impact modifiers in rigid PVC formulations, whereas the substantially fully hydrolyzed EVA resins do not.
U.S. Pat. No. 4,151,135 describes processes which permit the comminution of tacky EVA copolymers and tacky HEVA copolymers from the pellet and block form into the form of fine particles via dispersion in an aqueous medium with the aid of an alkali metal soap of a higher carboxylic acid as the dispersing agent, optionally in the presence of a water-soluble, substantially neutral salt, and the subsequent recovery of the particles from the dispersion as compaction-resistant dry powders by chemically modifying the dispersing agent in situ to provide a protective coating for the particles. In the preferred embodiment, the alkali metal soap dispersing agent is converted to the corresponding alkaline earth metal soap by reacting it with a suitable alkaline earth metal compound, e.g., calcium hydroxide. Alternatively, the alkali metal soap dispersing agent can be modified chemically by acidifying it with sufficient acid to release the corresponding carboxylic acid at least in part, which provides sufficient protective action to permit the dispersed copolymers to be isolated, dried, and handled as free-flowing powders. Higher carboxylic acid-protected powders, however, are inferior in compaction resistance to alkaline earth metal soap protected powders, but the acid coating is readily converted to alkaline earth metal soap coatings by reaction with sufficient alkaline earth metal compound. As an alternative, the acid-coated particles can be coated with alkaline earth metal soap, either performed or formed in situ. By including sufficient saponifying agent in the EVA dispersion step to saponify the combined vinyl acetate in the EVA, the corresponding HEVA can be obtained directly as the dry powder, after applying any of the above methods for providing protection, without the need for hydrolysis before dispersion. In general, the EVA and HEVA resin powders obtained in accordance with the foregoing procedures are made up of particles, usually spherical, of an average diameter ranging from about 20 microns up to about 500 microns, with the majority (80-90%) of particles being less than 250 microns. With the preferred dispersing systems, a substantial majority of the particles are less than about 150 microns, usually ranging from about 20 to about 150 microns with most of the particles being in the range of about 50 to about 100 microns. Nevertheless, a significant portion of the particles, typically 5-10 weight percent of the total weight of dispersed resin, will be in the range of from about 420 to 500 microns or greater and as such can be regarded as oversized and disadvantageous in various blending and thermoforming operations. It may be noted that oversized particles are usually not spherical but are for the most part elongated, fiber-like particles and as such, are prone to causing obstructions in processing equipment, a result of their poor flow characteristics. In addition, where, for example, blending of EVA and PVC powders are concerned, uniform distribution of the EVA in the PVC requires that the particles of both resins be substantially spherical and of approximately the same size. Accordingly, where significant quantities of oversized particles cannot be tolerated, it becomes necessary to remove them by screening and, if desired, recycle them to the dispersion process. Such secondary operations, screening and recycling, necessarily reduce the efficiency of the dispersion process and result in higher capital investment and operation costs.